In the modern era, society is becoming ever more dependent on access to, and the manipulation and storage of, increasingly vast amounts of data, at ever increasing speeds. Indeed, the ability to access large amounts of robustly stored data at a high speed is vital in many industries, as well as highly desirable for individuals when, for example, accessing the internet and the like. Access to information can promote freedom of choice, improve efficiency, drive innovation and economic development, and may overall lead to an improved quality of life.
Memory devices are typically semiconductor-based, integrated circuits for use by computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), non-volatile floating gate NOR/NAND flash memory, and dynamic random-access memory (DRAM).
Flash memory is a semiconductor device that utilises an electrically isolated floating gate within which charge is selectively stored. A conventional flash memory cell comprises a semiconductor substrate (usually silicon), which is doped to form separated source and drain terminals. A control gate terminal is also provided, with an electrically isolated floating gate disposed between the control gate and the substrate. A voltage applied to the control gate that is greater in magnitude than a threshold voltage enables current flow along a conductive channel, also known as an inversion layer, in the semiconductor substrate between the source and the drain terminals.
If charge is located within the floating gate, then the floating gate partially screens the control gate from the channel, thereby increasing the magnitude of the threshold voltage, ie the voltage at the control gate that is needed for current to flow through the channel. There are therefore at least two states of the device, a state in which charge is held in the floating gate and hence the device has a first threshold voltage, and a state in which no charge is held in the floating gate and hence the device has a second, lower threshold voltage. The state can be determined by applying an intermediate voltage to the control gate, ie a voltage that lies between the first and second threshold voltages, and sensing the current flow within the channel. The two states may be viewed as a bit, and thus the presence or absence of charge in the floating gate may provide a memory function for a device.
Due to the electrical isolation of the floating gate, which is typically achieved by placement of an oxide layer between the channel and the floating gate, and an oxide layer between the control gate and the floating gate, charge may be held within the floating gate for extremely long periods of time without the risk of charge being removed from the floating gate. Thus, flash memory is a non-volatile form of memory, which allows for robust storage of data.
Flash memory typically makes use of one of two mechanisms for the transfer of charge between the channel and the floating gate, namely Fowler-Nordheim tunnelling and hot-carrier injection (HCI).
Fowler-Nordheim tunnelling is a quantum mechanical effect relying on the tunnelling of electrons through a potential barrier. The probability of an electron passing through a barrier is greater for barriers of a smaller width, and thus, in order to facilitate the transfer of electrons between the channel and the floating gate, it is desirable to have as thin a layer of oxide as possible.
This, however, contradicts the requirement for non-volatility, which requires that the oxide layer is thick enough to prevent charge from leaking from the floating gate to the channel, and vice versa. The increased thickness that is necessary for non-volatility thus results in a lower probability of electrons being transferred between the channel and the floating gate, thereby increasing write and erase times.
Hot-carrier injection relies on the application of a voltage between the source and drain, which increases the kinetic energy of electrons in the channel such that they are able to pass through the oxide layer and into the floating channel. However, this requires a high voltage to be applied, which may in turn create a high electric field through the structure. Furthermore, increased voltages may result in electrons having increased kinetic energy to such a degree that, when electrons collide with the oxide layer, degradation of the oxide layer may occur, thereby limiting the number of times that electrons may be transferred (ie data may be written and erased) from the channel to the floating gate.
Various alternative methods to conventional flash memory have been proposed, for example the production of nitride trapping layers in SONOS or TANOS devices, yet none of these methods have been able to combine long term charge retention, ie non-volatility, with the fast write and/or erase times that are desirable.
There has now been devised a memory cell, method of manufacturing a memory cell, and a method of operating a memory cell, which overcome or substantially mitigate the aforementioned and/or other disadvantages associated with the prior art.